Baka, baka, baka… [stupid, idiot, … for the non-Japanese readers]
In setting up the Xcode project for GDAL, one of the GRASS libraries was left as a duplicate of another GRASS library. Thus the db drivers for GRASS wouldn't link or load no matter what I tried. (That means attributes couldn't be accessed from OGR or QGIS.)
This has been fixed in today's GDAL framework. The Xcode project has also been updated to match.